FSA OS-99 CSI stem review

Carbon wrapped alloy stem

Our rating 
3.5 out of 5 star rating 3.5
GBP £139.99 RRP
FSA OS-99 CSI Stem

Our review

Stiff, lightweight stem built to a high quality but pricey
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The heart of this stem is aluminium alloy – T6 heat-treated 7050 to be precise – forged and then machined away in parts, with a carbon layer around the outside. FSA says that rather than simply adding bling-appeal, its new CSI (Carbon Structural Integration) technology strengthens the stem, reduces brittleness and improves shock absorption. 


Corrosion-free titanium bolts hold everything in place, four sharing the load on the face plate to add a touch more confidence over a two bolt set-up. Hitting the scales at just 141g it’s a light unit, and that’s for our lengthy 130mm model. It’s a touch heavier than the non-CSI version and £20 more expensive so you’re paying for the extra strength. 

There is virtually no flex in use, though, so if you need something exceptionally taut up front this could be the lightweight option for you. Available in 80-130mm models, it can be run with a 6° rise or flip it for a 6° drop.


It comes with a 31.8mm clamp diameter for oversized bars and fits 1 1/8in steerers. 

Product Specifications


Name OS-99 CSI Stem
Brand FSA

Weight (g) 141
Angle 6
Steerer Size 1 1/8
Clamp/Bar Size 31.8
Bike Use Road
Stem Length/Reach Options 100mm 110mm 120mm 130mm 80mm 90mm